Experimental Study of Subcooled Pool Boiling around a Circular Rough Cylinder

Subcooling degree and surface roughness are two major parameters that have a considerable effect on boiling heat transfer. In the present study, the effects of subcooling degree on pool boiling heat transfer coefficient and surface temperature distribution are investigated experimentally. Tests are conducted for saturated and subcooled water with different subcooling degrees in the local atmospheric pressure (863 mbar) around a horizontal stainless steel cylinder with specific surface roughness. The test section is a pool with dimensions of 120×400×550 mm and test case is a circular cylinder with 80 mm length, 9 mm diameter and 0.794 μm average surface roughness. In this research, experiments are performed for the degrees of subcooling between 5.5°C to 45.5°C and for the heat fluxes between 0.31 kW/m32 to 125.62 kW/m. Results show that by increasing the degree of subcooling for a specific average surface roughness, average surface temperature is decreased and due to changes in the mechanism of heat transfer from nucleate boiling to natural convection, heat transfer coefficient is also decreased. In the region of natural convection, the variation of heat transfer coefficient with heat flux is low and when boiling process begins, this variation is more considerable. Furthermore, for lower heat fluxes (less than 5 kW/m), the temperature difference between upper and lower sides of the test case is less than 1°C which  increases for higher heat fluxes so that for more than 100 kW/m2,it reaches to 6°C.